1. Field of the Invention
The present invention relates to a diamond substrate used for a surface acoustic wave (SAW) device applied to high-frequency filters or the like, and to a SAW device.
2. Related Background Art
With recent tendency toward higher frequencies in the communication field, there are growing demands for development of SAW devices applicable in the high frequency (e.g., 2.5 GHz) region. (A typical example of the SAW devices is a SAW device in which interdigital electrodes (IDT: Inter-Digital Transducers) are formed on a piezoelectric layer, a surface acoustic wave is oscillated in the piezoelectric or a medium except for the piezoelectric by action of the interdigital transducer and the piezoelectric, and the surface acoustic wave is detected by the interdigital transducer and the piezoelectric.) In order to substantiate the SAW devices applicable in the high frequency region, it is necessary to narrow the electrode spacing of the interdigital transducers, or to increase the propagation speed of the surface acoustic wave. Since the interdigital transducers are normally formed by photolithography, there is a limit to miniaturization thereof. It is thus necessary to realize a medium with high propagation speed of SAW.
Since diamond has the largest elastic modulus among all materials, the SAW propagation speed is high in the medium of diamond. The conventional SAW devices applicable in the high frequency region were thus those in which the piezoelectric layer was formed on a thin film of diamond as a medium of SAW and in which the interdigital transducers were further formed on the piezoelectric layer.
The conventional SAW devices using the thin diamond film, however, had the problem of increase in insertion loss, because the SAW propagation loss was large in the diamond film.
The diamond substrate for SAW device is desired to have the high propagation speed of SAW (preferably, 9000 m/s or higher), in order to enable use in the high frequency region.
The present invention has been accomplished in order to solve the above problem and an object of the invention is to provide a SAW device diamond substrate and a surface acoustic wave device with a diamond film realizing the high propagation speed and small propagation loss of SAW.
In order to achieve the above object, a SAW device diamond substrate according to the present invention is a diamond substrate for surface acoustic wave device comprising a substrate, and a diamond film formed on the substrate, wherein a hydrogen content of the diamond film is in the range of not less than 1% nor more than 5% in atomic percent and an arithmetic mean roughness (Ra) of a surface of the diamond film is not more than 20 nm.
When hydrogen is taken in during the forming process of the diamond film, hydrogen intervenes in the bond between carbons to lower the crystallinity of the diamond film. Particularly, when the hydrogen content is approximately 30%, the diamond film tends to become of amorphous structure like diamond-like carbon (DLC). As the hydrogen content of the diamond film increases, the SAW propagation speed becomes lower because of decrease of hardness (decrease of elastic modulus) of the diamond film.
As the hydrogen content of the diamond film decreases on the other hand, the surface acoustic wave is scattered more at grain boundaries because of high crystallinity in the diamond film, so as to increase the propagation loss.
The rougher the surface of the diamond film, the more the surface acoustic wave is scattered, to increase the propagation loss.
Inventor conducted elaborate research and found that the preferred propagation speed and propagation loss of SAW were simultaneously achieved under the conditions that the hydrogen content of the diamond film was in the range of not less than 1% nor more than 5% in atomic percent and the arithmetic mean roughness (Ra) of the surface of the diamond film was not more than 20 nm.